1. Field of the Invention
The present invention relates generally to an infrared circular dichroism measuring apparatus and an infrared circular dichroism measuring method, and more particularly to a reduction of the measuring time and an improvement of the measuring accuracy.
2. Prior Art
For many chemical substances, findings relating to their absolute structures and stereo-structures are extremely basic and essential information. X-ray structural analysis, circular dichroism spectrum analysis, etc. can be referred to as means for analyzing the chirality of physiologically active substances such as, for example, drugs, poisons and biological substances. The circular dichroism spectrum analysis is especially widely used as important and essential means for the studies in this field since it is relatively easy to handle.
Molecules having mirror-image-asymmetric molecular structures have a property that the magnitudes of their absorption for clockwise-circular-polarized light beams and counterclockwise-circular-polarized light beams are different. This property is called circular dichroism. Many physiologically active substances have the optical activity and information about their molecular stereo-structures can be obtained by measuring their circular dichroism. The information obtained in this way is used in identifying their structures.
Infrared circular dichroism measurement can be used especially in identifying the structures of optical active substances that do not have absorption in, for example, UV and visible light regions.
FIG. 5 is a schematic representation of a conventional infrared circular dichroism measuring apparatus. In the infrared circular dichroism measuring apparatus 1 shown in the figure, an interference light beam is generated by passing an infrared light beam radiated from an IR light source 2 through a Michelson interferometer 3. On the optical path of the interference light beam, a polarizer 4, a photoelastic modulator (PEM) 5, a sample 6 and a detector 7 are provided. The interference light beam becomes a linearly polarized light beam by being passed through the polarizer 4 and the linearly polarized light beam is converted by PEM 5 into a circularly polarized light beam that is generated in clockwise and counterclockwise alternately at a predetermined modulation frequency. The modulation frequency of PEM 5 is controlled by a PEM controller 8. The infrared light beam modulated in this way is detected by the detector 7 after passing the sample 6. A detector that can respond quickly to be able to cope with the PEM frequency around 50 KHz such as PC-type MCT detector is used as the detector 7.
At this moment, a signal shown in FIG. 6 is detected by the detector 7. That is, for example, when a clockwise and counterclockwise circular polarized light beam is generated at a modulation frequency of 50 KHz by the PEM controller, the signal detected after its passing through the sample must have an alternate-current component modulated at the modulation frequency of PEM 5 because the sample that is an optical active substance has different magnitudes of its absorption for a clockwise circular polarized light beam and a counterclockwise circular polarized light beam. Then, a doubly modulated signal in which the alternate-current component is superposed on the modulation (at lower 3 KHz) by the interferometer 3 is detected.
Interferograms produced by each of the clockwise and counterclockwise circularly polarized light beams are extracted from the signal detected at the detector 7 by, after being amplified at a pre amplifier 9, passing through a band pass filter 10, a lock in amplifier 11 and a data acquisition circuit 12. That is, the band-pass filter 10 passes therethrough only a signal in a predetermined frequency band containing the modulation frequency of PEM 5 and the lock in amplifier 11 lock-in-detects the component having the modulation frequency of PEM 5 using a synchronized signal. At this moment, the detected component is sampled with a predetermined time constant (the time period necessary between the moment the lock in amplifier outputs a measured signal and the moment the amplifier outputs the next measured signal) and an alternate current having an intensity variation of the modulated component at lower 3 KHz by the interferometer 3 is obtained.
On the other hand, an interferogram by infrared absorption is extracted by, after amplifying the signal detected at the detector 7, passing the signal through a low pass filter 13 and the data acquisition circuit 12.
Based on the interferograms produced from each of the clockwise and counterclockwise circularly polarized light beams and the interferograms produced from the infrared absorption extracted as above, a Fourier transformation is conducted at a host PC 14 to calculate a circular dichroism spectrum that is the difference spectrum (ΔA) between absorption spectra produced from each of clockwise and counterclockwise polarized light beams.
In an infrared circular dichroism measurement, the intensity of a signal obtained is weak and, therefore, a measurement is conducted by radiating a multi-wavelength infrared light beam to a sample at the same time using a Fourier transform infrared spectrometer. Therefore, this measurement has the following problems.
First, at the central wavelength of the PEM, i.e., a narrow region around the light beam wavelength that has the most high efficiency for generating a circularly polarized light beam, a circularly polarized light beam is efficiently generated, however, in a wavelength region away from that narrow region, the efficiency for generating a circularly polarized light beam is reduced and the measurement efficiency is degraded.
Furthermore, the alternate-current component modulated at the modulation frequency of PEM is very faint (the absorbance A is ordinarily around 1, however, it is around 10−4–10−5 for a circular dichroism measurement) because the difference between the absorption spectra produced from each of clockwise and counterclockwise circularly polarized light beams is very small. Therefore, a plurality of measurement are necessary to improve the S/N ratio and a measurement can not be conducted in a short time (for example, an integration for one (1) to two (2) hours is necessary).
On the other hand, the intensity of the light beam detected is very strong because of light beams each having a wavelength different from each other are contained in the light beam at the same time. Therefore, when a PC-type MCT detector that can respond quickly to be able to cope with the PEM frequency around 50 KHz is used as the detector, a signal in proportion to the light beam intensity can not be output in terms of the regions where the detected light beam intensity is too strong and a non-linear response is occurred, resulting in an adverse influence on the measurement accuracy.
Because the intensity of a modulated signal (interferogram) produced from an interference light beam is rapidly attenuated as shown in FIG. 6, the tail portion where the intensity is weak is influenced strongly by noises. Therefore, the dynamic range in the portion is restrained due to the SIN ratio originated in the influence and quantization error produced during an AD conversion.
In terms of S/N ratio improvement, it is preferable to extend the time constant of the lock in amplifier. However, when sampling of a signal having an intensity variation of the component modulated at lower 3 KHz by the interferometer as shown in FIG. 6 is assumed, it is necessary to measure with a time constant of 1 m·second or shorter and the S/N ratio is limited to a specific extent because the component modulated by the interferometer can not be obtained if the time constant is extended too long.
Furthermore, there is another problem. Since a circular dichroism spectrum is obtained corresponding to the position of an absorption peak, it is possible to know which molecular vibration originates a specific circular dichroism spectrum. However, it is the current state that the relation between information on the molecular structures and the shapes of the circular dichroism spectra have not been made sufficiently clear for infrared circular dichroism. There are up to several absorption peaks in UV and visible light regions, while, in contrast, there are a large number of absorption peaks in the so-called finger-print region of infrared and, therefore, their assignment is very complicated. In above respects, means for clarifying the relation between circular dichroism spectra of specific absorption bands and molecular structures have been sought.
The present invention was conceived in view of the above problems involved in the prior art and its object is to provide an infrared circular dichroism measuring apparatus and an infrared circular dichroism measuring method that achieve improvements in the measuring time and the measuring accuracy.